View Under the Hood:

Lumus substrate-guided optics may transform mobile viewing

By Bernard Cole
iApplianceWeb
(05/15/06, 7:54 AM GMT)

For most of their brief lifetime as the primary connection of the average consumer to the Web as well as to a variety of video entertainment, mobile and portable electronics devices have presented a challenge to designers as tough as trying to pass a camel through the eye of a needle.  

The problem facing makers of such handheld devices is that if they make the display big enough to view video or photographs with any degree of resolution and detail, the resulting device is no longer very portable or mobile. If they use a display appropriate to the size and mobility of the device, they have a viewing area that is too small and not very readable.

Manufacturers are trying a variety of alternatives: one is to make the displays wider and shorter and built on a swivel so that they can be used simply as a telephone viewer in the vertical model and a widescreen video viewer in the other.  Another possible solution is to keep improving the resolution of the small screen, hoping the brightness and fine detail of the display will make up for the small viewing area.

Alternatively, a number of manufacturers have emerged with ‘microdisplays’ that provide a virtual viewing area of 17 inches from a foot or so away. They are based on various semiconductor derived fabrication technologies that provide a heads-up, near-eye virtual display alternative which positions an extremely small viewing screen close to the eye, tricking the viewer into thinking they are seeing a large regular monitor or TV screen from a foot to several feet away.

But while they have been able to drive the costs down enough to provide a viewing alternative in some of the new multimedia-based PDAs and mobile devices, such display alternatives also have problems of physics to face that are almost, but not quite, as difficult as the display versus mobile/portable size problem.

Most of the problems have to do with the relationship between the angular extent of the image viewed, called the field of view (FoV) to the left, right, up and down. The human eye’s comfortable range of motion in those four directions, called the eye motion box (EMB), and the eye’s distance from the flat surface upon which, or from which, the image is displayed, are called its ‘eye relief’.

Even with the most advanced semiconductor image electronics, most heads-up, near-eye visual systems are limited by the fact that they must use classical, and bulky, optical components – reflectors, refractors, prisms and lenses. This often results in head-mounted virtual viewers that extend several inches out from the front of the face, looking much like a pair of binoculars attached to the head, for a viewing experience roughly comparable to looking at a 17-inch, 800 x 600 desktop computer monitor with a 12-point plain text font from about a foot or two away.

For the best eye relief distance and best viewing, the head-mounted near eye is about 30 millimeters, or about 12 inches. This distance can be reduced somewhat by using a folded optical system that trades lateral left/right distance for distance to viewing image, and by limiting the field of view up and down, left and right.

The problem is that using such classical optics solutions reduces the amount of information that can be viewed on the virtual display, because, in effect, instead of a 17-inch screen, the field of view that can be seen is the equivalent of a 10-inch diagonal screen.  

Resolving the dilemma

A startup called Lumus Technology, based in Rehovot, Israel, has solved this problem by replacing the classical optics with a thin plastic or glass piece it calls the LOE (Lumus Optical Element) which makes use of what is called ‘substrate-guided’ optics. As shown in the figure below, in this approach, an image from the semiconductor imager is captured within the plate’s substrate guide optical channels and moved the appropriate distance within the plate ensure the optimum FoV, EMB and eye relief distance.

Each portion of the internal reflective surfaces is carefully designed to reflect out of the LOE plate only a predetermined range of optical components, such that different portions of the image are coupled out to the viewer’s eye as a properly reconstructed image. The result is an effective exit image that is much larger than the original with a wider field of vision and eye motion box but with a much shorter eye relief distance.

The design of the plate’s internal reflective optics decouples the field of view and the eye motion box dimensions from the overall dimensions of the physical optics. The result is a 4-mm thick LOE plate with a FoV of about 40 mm and an EMB greater than 15 cm. This provides a virtual view, with no limitations as to viewing area and high resolution, in a fraction of the volume required by current heads-up, near-eye viewers.

Lumus plans for LOE

Lumus is targeting a number of markets with its light see-through personal displays that can be attached to various headgears or worn like a pair of glasses, with different resolutions and corresponding fields-of-view to accommodate different user requirements: head-worn displays (HWDs); wearable smart terminals (WSTs); handheld device displays (HDDs); and heads up displays (HUDs).

In HWDs, images generated by a computer (VGA interface) or from a video source (C-video interface) generate a see-through virtual image, while the normal field of vision of the real world is unobstructed. The image source and the LED-based illuminator are mounted on top of the head, while the electronics are located in a small box attached to the headgear and connected by a cable to the head unit. The box includes also batteries, manual illumination control, and volume control for the optional audio.

The WSTs the company is working on consist of a Head Worn Display combined with a pocket PC with wireless communication links (such as WiFi, IR or WAP), in which the displayed image can be acquired from any source, including a remote camera or remote PC which transmits the image via WiFi, IR or cellular communications.

There is an optional two-way audio system used for communication, signaling and voice activation control of the application, enabling bi-directional audio and video streams as well as Internet connection.

There is an also optional wearable pointing device, to navigate through an application, and an optional camera to obtain visual data on the environment for local processing and for transmitting visual information to a remote location. Another optional element is a head-location tracking system. This can be an independent tracking sensor or software operated tracking using the visual signal from the camera.

Further out, Lumus is currently developing an HDD application that projects an A5-size image but still fits inside a cellular phone. In this configuration the device is hand-held (HHD) and the image, which ‘materializes’ at a distance behind the device, is viewed through a window similar in size to that of a common cellular-phone display.

The LOE the company is working on offers full-color performance, high-resolution and an image that is not sensitive to the relative location of the viewer’s eye, with a 1.5” window projecting a 15” virtual image.

Addressing the automotive and personal mobile device markets, Lumus is also working on an alternative HUD configuration for pilots and drivers. The personal display will project the car’s or plane’s instrument data in front of the driver/pilot, in his field of vision. Using this HUD, the driver or the pilot will not have to take his eyes off the route and to look down to see how the vehicle/plane is performing.

The potential for a $100 Head's Up Viewer

While it offers viable ways to reduce the head-mount size relative to the virtual display size and resolution, the company has a number of competitors who have developed alternatives based on more mature technologies.

Many of these companies have been working on these alternatives since the mid-90s, paying for their research by focusing on targeted markets within military and aerospace, and specialized applications in engineering and maintenance, which require a more portable means of viewing documentation, usually only available in digital form on a computer.

At recent Consumer Electronics Shows this year and last, a number of eyepiece vendors had offerings ranging from $750 to $1500, including  Inginio, IO Display, Incuiti, Liteye, and WorldViz. On the technology side, the four most active companies are  Brillian, eMagin, eLCOS, and Kopin, each with prototypes and reference designs for technologies they’re championing.

The most mature technology is Kopin’s transmissive LCDs. They’re designed to transmit light from a backlight through the liquid crystal layer and out through the front to the user’s eye. Brillian and eLCOS are now using LCOS (liquid crystal on silicon), originally developed for large screen flat-panel displays, for headset and eyeglass viewer displays. Organic LEDs, championed by eMagin, generate self luminous displays that don’t require backlighting.

Most of these companies are also targeting the under $500 segment in both the portable media player and 3G/4G mobile markets, with high-quality color displays of about 720 by 560 pixels (CIF) and 180 by 144 pixels (QCIF), capable of resolving video displays at roughly 30 frames a second. Because Lumus LOE can be made for either glass or plastic, it is an ideal replacement for the conventional optical systems used in these current head’s-up devices.

Lumus executives project that the mass production of a mid-range LOE suitable for use in a consumer-oriented heads-up near-eye viewer could be significantly less than $10, but with much better visual quality. If so, expect heads-up viewers much closer to the $100 or less price point of current low-end PDAs, portable media players and cell phones, and a fraction of the cost of the next-generation multimedia-optimized devices.

To learn more, go to www.lumusi-optical.com.

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